1. Field of the Invention
The present invention relates to channel equalization, and more particularly, to an apparatus for channel equalization and method thereof, by which a digital signal received via a plurality of antennas is equalized.
2. Discussion of the Related Art
Generally, a digital transceiver system maps digital information (e.g., voice, data and video) of a transmitter into symbols, converts each of the symbols to an analog signal proportional to a size or phase, and then transmits the analog signal to a receiver over a transport channel.
In doing so, interfering with signals passing through the transport channel of multi-path, the signal arriving at the receiver is severely distorted. Hence, an equalizer is needed for channel compensation to restore an original signal from the distorted received signal.
Currently, as an equalizer mostly adopted by a receiver for a single carrier transmission system such as the U.S. terrestrial broadcasting, there is a decision feedback equalizer.
The decision feedback equalizer, which has less noise increment and includes an infinite impulse response (IIR) filter, is advantageous in compensating signal distortion due to a time delay corresponding to length of the filter but is disadvantageous in a problem of instability due to incorrect decision.
To compensate such a disadvantage, a predictive decision feedback equalizer (pDFE) provided with a linear filter only has been proposed, which is explained with reference to FIG. 1 as follows.
FIG. 1 is a block diagram of a predictive decision feedback equalizer according to a related art.
Referring to FIG. 1, a predictive decision feedback equalizer consists of a linear filter equalizer 111, a noise predictor 114, an error generator 112, and a decision device 115.
The above-configured predictive decision feedback equalizer performs linear equalization on an input signal u(n) by the linear equalizer 111 to obtain a signal x(n) and removes a noise amplified in the equalization step by n(n) predicted by the noise predictor 114 to obtain a final result of y(n).
In doing so, the predictive decision feedback equalizer regards a difference between a value d(n) resulting from deciding y(n) as one of predefined values and the equalized value x(n) as an equalization error e(n) and then performs adaptive equalization.
The noise predictor 114 predicts a noise with the equalized x(n) and the d(n) to find the n(n).
The predictive decision feedback equalizer acts as a substitute for noise reduction obtained in using a decision feedback filter. Yet, the predictive decision feedback equalizer needs a longer filter to equalize distortion of an area such as a decision feedback equalizer because of using the linear feedforward filter only.
In case of receiving broadcasting in the urban center or room, signal interference brings about strong distortion. Hence, a feedforward filter needs a considerably long length to linearly equalize the strong distortion.
If a linear equalizer is implemented in a time domain, hardware complexity is raised considerably high. If the LMS (least mean square) method is used as adaptive algorithm, a filter length is elongated so that a channel variation speed that the equalizer can follow up is more lowered.
Hence, a method enabling the linear equalizer in a frequency domain has been proposed, which is shown in FIG. 2.
FIG. 2 is a block diagram of a frequency domain linear adaptive equalizer according to a related art.
Referring to FIG. 2, an input data u(n) is overlapped by an overlap unit 211 and a signal U(k) of a frequency domain is found using a FFT (fast Fourier transform) unit 212. In this case, ‘k’ is a frequency index corresponding to 1˜N when time data is transformed into a frequency data using N-point FFT.
The data U(k) of the frequency domain passes through a conjugate operation unit 213 and a power normalizer 214 to be converted to U′H(k). In this case, conjugate operation and power normalizing operation are carried out regardless of operational sequence. So, the conjugate and power normalization operations can be simultaneously carried out.
The power normalizing operation is to achieve normalization by distributing data with power corresponding to each frequency index of signals. By the power normalizing operation, a different coefficient update quantity is given to each frequency index.
Meanwhile, zeros amounting to an overlapped quantity of the data u(n) is added to an input error e(n) of time domain by a zero padding unit 226. And, the error e(n) is then transformed into an error value E(k) of the frequency domain by an FFT unit 225.
The error value E(k) is multiplied by U′H(k) in a multiplier 215. The result is multiplied by a step-size μ to be added to a coefficient W(t)(k) of a current time t, whereby a coefficient W(t+1)(k) of a next time (t+1) is found.
Thereafter, the data U(k) of the frequency domain is multiplied by an equalizer coefficient W(k) for equalization to be outputted. And, the corresponding output is inversely transformed into to a value in a time domain by an IFFT (inverse fast Fourier transform) unit 223. A save unit 224 having received an output value from the IFFT unit 223 discards overlapped data but takes valid data to obtain a final result x(n).
A case that there is no coefficient update restriction is taken as an example for the above-explained channel equalization process. Yet, if coefficient update restriction is put on an equalizer, an operation procedure within a dotted line in FIG. 2 is additionally needed. In the configuration of the equalizer having the coefficient update restriction, a value resulting from multiplying the error E(k) in the frequency domain by the data U′H(k) is transformed into a value in the time domain through IFFT operation. A coefficient update range is limited in the time domain (a coefficient to be updated in the time domain remains intact but the rest is turned into zero). The corresponding value is then moved to the frequency domain through FFT operation to generate a size to update the coefficient in the frequency domain.
Meanwhile, in order to equalize a signal experiencing a dynamic channel having fast variation, an equalizer having a fast adaptive speed is needed. There exists a difference between a channel speed an equalizer can converge and a traceable speed after the convergence in general. To raise the convergence speed, a method of initializing an equalizer according to a channel status is needed. So, an initialization-enabling frequency domain linear adaptive equalizer that enables initialization has been proposed, which is explained with reference to FIG. 3 as follows.
FIG. 3 is a block diagram of an initialization-enabling frequency domain linear adaptive equalizer according to a related art.
Referring to FIG. 3, in an initialization-enabling frequency domain linear adaptive equalizer, information additionally needed to initialize a coefficient of an equalizer is channel impulse response (CIR).
Hence, c(m) is found by presuming a channel before equalizing initial reception data of signal reception. And, zero is padded into the c(m) by a zero padding unit 312 to correspond to a data FFT size. In this case, ‘m’ is an index according to a presumed channel response time.
An output value of the zero padding unit 312 is transformed into a value of the frequency domain by an FFT unit 314 to find a frequency domain response C(k) of the channel. The C(k) becomes an initial coefficient value W(0)(k) of an equalizer by finding a frequency response of a inverse channel using a ROM 315. A coefficient is then initialized using the initial coefficient value. An adaptive equalization procedure after initialization is executed by the same manner explained in FIG. 2.
However, the related art equalizer linearly equalizes the signal according to the antenna path using one antenna only and removes the noise amplified in the equalizing process using the noise predictor. So, like the case that a signal reflected via various paths of the urban center, the indoor space and the like is received, noise amplification is raised in the channel having severe distortion. Hence, it is difficult to compensate signal distortion in the related art equalizer.